The primary function of a fuel supply system is to provide a sufficient flow of fuel to an engine to meet its operating demands. In continuous combustion engines, such as those typically used in a turbine aircraft engine, the fuel supply system typically employs high pressure positive displacement fuel pumps. Such high pressure positive displacement fuel pumps are typically driven via a gear box coupled to the main turbine engine. As such, the pump flow rate in such a fuel supply system is directly proportional to the engine speed, i.e. the greater the speed of the turbine engine, the greater the fuel flow rate from the high pressure positive displacement fuel pump.
While such a configuration is successful at providing a fuel flow rate to the turbine engine that allows the engine to meet its operational demands, these operational demands do not always require such a high flow rate of fuel. Indeed, there are many engine operating conditions at which the engine flow demand for fuel is significantly less than the high pressure flow supplied by the positive displacement pump. Because the excess flow of fuel not required by the engine must flow somewhere, it is typical that the excess high pressure fuel is bypassed back to the low pressure inlet of the high pressure positive displacement fuel pump. Unfortunately, raising the pressure of the excess fuel flow and returning that excess flow back to the low pressure inlet of the pump effectively wastes the energy that had been added to the fuel as it was supplied to the turbine engine. This wasted energy is realized as a heat input into the fuel, and often results in undesirably high fuel temperatures in some operating conditions.
In an attempt to overcome this thermal issue, many fuel control system designers have implemented a dual pump system. In such a system, two fuel pumps are utilized to supply the engine with a flow of fuel that is more closely related to the actual fuel demand by the engine. This is accomplished through a switching system whereby the fuel flow from the second pump may be added to the fuel flow from the first pump only when the engine demands exceed the capability of the first pump for providing sufficient fuel flow. In this way, during low fuel demand operating modes, only the flow from one of the fuel pumps is supplied to the engine, and therefore the potential for excess flow that must be returned to the pump inlet is reduced. While during such low fuel demand operating modes the second pump continues to operate, such dual pump systems typically return the output of the second pump to its input directly such that the second pump operates with a minimum pressure differential across it. However, as the fuel demand from the engine increases beyond the capability of the first pump, such dual pump systems typically add the flow from the second pump to that of the first pump so as to ensure an adequate supply of fuel to the engine.
While such typical dual pump fuel supply systems have addressed the thermal increase issue common with the use of a single high pressure positive displacement fuel pump, the addition of the fuel flow from the second fuel pump to that of the first often results in engine burn flow disturbances that are associated with the switch between the pumping modes. In modern high efficiency turbine engines, such burn flow disturbances are unacceptable. Various control mechanisms including bypass valve position sensing, solenoid control, engine speed sensing, etc. have been tried in an effort to minimize the fuel flow disturbances resulting when the fuel supply system is transitioned between the single and dual pump modes of operation. While some of these systems have met with success, each require additional control signals from the engine control unit, fuel metering unit, or other components within the overall turbine engine control system. This increases the complexity, and therefore reduces the reliability, of the overall system.
There is need in the art, therefore, for dual pump fuel supply system that reduces or overcomes the problems existing in the art. Embodiments of the present invention provide such a dual pump fuel supply system and switching methods. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.